From DE-A-4340165 and DE-A-4340173 such methods are known, by means of which, for example, cotton or wool in the form of flocks polluted to a greater or lesser extent with impurities may be freed of said impurities. With said methods it is possible to distinguish between external impurities, which relate to different material, and internal impurities, which relate to the same material but in a different state or a different color. Internal impurities are, for example, cotton or woollen fibers which are partially rotten, agglutinated or contaminated. External impurities are stones, soil, glass, stalk residues, leaves, packaging material, hair, feathers etc. Whereas crude impurities are removed in the known spinning preparation devices, impurities which are more difficult to separate are, according to the known methods, to be detected and removed from the stream of loose material. To said end, the fibers or flocks are conveyed continuously past color sensors which are to detect impurities. Material containing constituents, to which the color sensors have responded, is then removed.
A perceived drawback of such known methods is that many impurities are not detected thereby. one reason is, for example, that impurities, in order to be detected, have to vary in color to a relatively great extent from the textile fibers or the background, which is not always the case. Such known methods do not operate very selectively.